1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition, and particularly to a piezoelectric ceramic composition useful as a material for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators.
2. Background Art
Piezoelectric ceramic compositions predominantly comprising lead titanate zirconate (Pb(Ti.sub.x Zr.sub.l-x)O.sub.3) or lead titanate (PbTiO.sub.3) have been widely used for piezoelectric ceramic elements such as piezoelectric ceramic filters. In the production step for these types of piezoelectric ceramic compositions, lead oxide is generally used. However, vaporization of lead oxide causes deteriorated uniformity in characteristics of the produced elements.
In contrast, piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate represented by a formula such as (K.sub.l-x-y Na.sub.x Li.sub.y)NbO.sub.3 do not cause the above problem since they contain no lead oxide. Some such compositions comprising potassium sodium lithium niobate have a high electromechanical coupling coefficient K.sub.p and are considered to be promising materials for producing piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators.
However, the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate have a relative dielectric constant lower than that of lead titanate zirconate or lead titanate. Therefore, when they are used as materials for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic oscillators, impedance-matching with a circuit including the ceramic elements is poor and circuit design sometimes become difficult.
When a piezoelectric ceramic composition is used in a high-frequency region, the following problems arise. Since a conventional piezoelectric ceramic composition predominantly comprising lead titanate zirconate generally has a relatively high relative dielectric constant (approximately 1000-2000), impedance decreases in the high-frequency region of, for example, more than 100 MHz to cause difficulty in use thereof in such a high-frequency region.
In contrast, piezoelectric ceramic compositions predominantly comprising lead titanate (PbTiO.sub.3) generally have a relative dielectric constant of approximately 200, which is lower than that of the above piezoelectric ceramic compositions predominantly comprising lead titanate zirconate. Therefore, the compositions comprising lead titanate (PbTiO.sub.3) are known to be useful in the higher high-frequency region. However, an even lower relative dielectric constant is desired in consideration of use in a higher high-frequency region.
In addition, piezoelectric ceramic compositions predominantly comprising lead titanate zirconate or lead titanate have a resonance frequency of vibration in a thickness direction as low as approximately 2000-2500 Hz.multidot.m. Thus, when such a piezoelectric ceramic composition is processed into a thin slice so as to form a vibrator, the vibrator must be used in a limited frequency band.
In contrast, some of the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate represented by a formula such as (K.sub.l-x-y Na.sub.x Li.sub.y)NbO.sub.3 have a resonance frequency of vibration in a thickness direction as low as approximately 3000-3500 Hz.multidot.m and a relative dielectric constant of approximately 100, which is lower than that of lead titanate. Therefore, some of the compositions are known to be used as a material which has characteristics more advantageous than those of lead titanate zirconate or lead titanate in consideration of use thereof in the high-frequency region.
However, the piezoelectric ceramic compositions predominantly comprising potassium sodium lithium niobate have a large temperature-dependent factor of resonance frequency of vibration in a thickness direction as high as approximately 150-300 ppm. This factor is referred to as fr-TC, and represents an important characteristic of a material for piezoelectric ceramic filters and piezoelectric ceramic oscillators. Therefore, the piezoelectric ceramic compositions have not yet been used widely in practice, as compared with lead titanate zirconate, lead titanate, etc.
The above-described temperature-dependent factor of resonance frequency of vibration in a thickness direction represented by fr-TC is calculated from the following equation: EQU fr-TC=(fr.sub.max -fr.sub.min)/(fr.sub.20 .multidot.100)
wherein fr.sub.max represents the maximum resonance frequency of vibration in a thickness direction within a temperature range of -20.degree. C. to +80.degree. C.; fr.sub.min represents a minimum resonance frequency of vibration in the thickness direction within a temperature range of -20.degree. C. to +80.degree. C.; and fr.sub.20 represents a resonance frequency of vibration in the thickness direction at 20.degree. C.